Research On The Switching Control Strategy Of Microgrid Islanded And Grid-connected Modes

As an effective method to exploit renewable energy and improve the reliabilityand flexibility of power supply, microgrid is a new way to solve environmentalpollution and resources shortage problems. Microgrid has islanded andgrid-connected modes. The seamless switching between the two modes has veryimportant research value, because it is the key to provide continous power supplyfor load, and improve power qulity, reliability and flexibility. This paper researcheson the control strategy of mocrogrid to make the switching process seamless.A microgrid system structure is designed, which is based on AC-bus and usesPV distributed generation. The topology structure of PV inverter and its parametersare designed, and its mathematical model is established. The reason why traditionalcontrol stategy will cause impact during modes switching is analized, and a newcontrol strategy is proposed for the microgrid. Based on droop theory, PV inverter isvotage-controlled during islanded operation, grid-connected operation and modesswitching. The switching of control strategies is avoided, thus the impact isessentially restrained.Firstly, the islanded control strategy is proposed. Droop equations adapt toresistive impendence are used to decouple active and reactive power, since theelectrical wires of this microgrid is under low votage. The system voltage andfrequency are stably controlled and reliable power supply is provided for load. Then,the grid-connected control strategy is proposed. Power flow of grid-connected PVinverter which is voltage-controlled is analized. Droop control can also be used ingrid-connected mode. Power loop and DC-bus voltage loop are added to droopcontrol method, so the power of inverter cannot be influenced by grid fluctuation,the power control ability of PV inverter is strengthened, PV inverter can outputmaximum active power into grid and the DC-bus voltage is stable. Lastly, modesswitching control strategy is proposed and pre-sychronization is accomplishedbased on droop theory so that the switching process is seamless.The proposed control strategies are analized under different conditions throughsimulation model. The experiment platform is established to verify the controlstrategies furthermore. The experiment results show that the inverter can operatestably in both islanded mode and grid-connected mode. Meanwhile, the seamlessswitching between the two modes is accomplished so that there’s no impact duringthe process and the load is provided continous power.